In motor vehicle on-board electric power systems so-called load dumping plays an important role in the specification of the requirements made of the power transistors which are used in the vehicle on-board electric power system.
This load dumping occurs if the connection to the car battery fails in the motor vehicle. The charge current which is made available by the dynamo continues to flow over a time of several hundred milliseconds and has to be absorbed or taken up by the automobile electronic system until a regulator responds and assumes complete control of the charge current from the dynamo of the motor vehicle.
However, in this time, a load current which is stabilized by means of load resistors to a typical current density of, for example 50 A/cm2 flows across the power transistors, as illustrated in FIGS. 1A and 1B. In this respect a zener diode arrangement ZDAZ is shown in FIG. 1A, and a zener voltage VZ1 between the gate terminal G and the drain terminal D is shown in FIG. 1B.
In this regard, the part of the conventional vehicle on-board electric power system 10 shown in FIG. 1A has three inputs and outputs of a logic circuit 3 with ON for an input voltage, IS for a sensor output current and SEN for a sensor enable voltage. The current which is fed into the logic circuit 3 via IN is limited by an input resistor RIN, and the input current from the input SEN is limited by a sensor enable resistor RSEN. In addition, the inputs IN and SEN of the logic circuit are protected against overvoltages by a zener diode arrangement ZDESD which, in the case of an overvoltage, discharges the currents limited by RIN and RSEN to an internal ground GNDi. Furthermore, a current which is limited by a grounding resistor RGND can be discharged via the zener diode arrangement ZDL to ground GND via the grounding resistor RGND when overvoltages occur at the on-board electric power system supply voltage terminal VBB.
In the case of load dumping, a generator which is not shown in FIGS. 1A and 1B firstly builds up a high cut off voltage at the power transistor 2, wherein, when the zener voltage VZ1 shown in FIG. 1B is exceeded, the zener arrangement shown there becomes electrically conductive so that a further increase in the cut off voltage switches on the gate G, i.e. before current can flow through the power transistor 2 from the source power electrode to the drain power electrode or vice versa depending on the conduction type of the power transistor 2. This current which is provided by the generator has to be discharged for some time (for example several 100 ms) at a high voltage from the power transistor 2 as forward current across the output terminal VOUT and the load 4 and in the process it heats the power transistor 2.
In order to avoid zener clamping at the maximum occurring load dump voltages of approximately 40 V in a passenger car on-board electric power system and approximately 58 V in a truck on-board electric power system, the minimum zener clamping voltages are selected by the power transistors in the passenger car on-board electric power system as, for example, typically 42 V and in the truck on-board electric power system as, for example, typically 60 V. The concept of active zener clamping, as shown in FIGS. 1A and 1B, has, however, the following decisive disadvantages:                1. all the other components which are connected to the voltage network must likewise withstand these high voltage requirements since the overvoltage pulse is not attenuated;        2. when a load dump overvoltage occurs, there is no flow of current in the passenger car up to approximately 40 V, i.e. no energy is extracted from the overvoltage pulse so that the overvoltage persists over a long time period;        3. a significantly higher value has to be selected for the breakdown voltage VDS of the semiconductor technology used than the minimum guaranteed zener clamping voltage taking into account variation, temperature drifts etc. This has an adverse effect on the chip costs. Necessary technology voltages are thus obtained for passenger car applications with 60 V and for truck applications with 80 V, which is significantly above the typical maximum load dump voltages of 40 V in a passenger car on-board electric power system or 58 V in a truck on-board electric power system;        
4. the higher the active zener clamping voltage the higher also the absorbed power if the power switch goes into the clamped state. This leads to a more rapid increase in temperature and to earlier damage to the power transistors.
Documents U.S. Pat. Nos. 5,115,369 and 5,365,099 disclose such an active zener protection system in which a multiplicity of zener diodes are integrated monolithically on the semiconductor material of the power transistor. This solution has the disadvantage that the required chip area is significantly increased, thus making the fabrication costs considerably higher. As a result, the installation volume of the power transistors is also disadvantageously increased. The reliability of such highly integrated power transistors requires an increased expenditure on analysis, as is known from the document by A. Castellazzi et al. “Reliability Analysis and Modeling of Power MOSFETs in the 42 V PowerNet”, IEEE Transactions on Power Electronics, Vol. 21, No. 3, May 2006, pages 603-612.
In this context, both the logic component and the power semiconductor component have until now been protected by means of the active zener clamping, i.e. from a specific zener clamping voltage the protection structure starts to conduct with a relatively low internal resistance so that a further rise in voltage at the semiconductor component is prevented. The level of the zener clamping is selected here such that only the high and short dynamic overvoltages are limited, but not the static overvoltage increases such as occur in the event of the abovementioned load dump pulse in motor vehicle electronics.
Clamping in the static state means a high power loss, specifically the product of the clamping voltage times the current, and would under certain circumstances thermally destroy the protection structures. This is particularly critical for the clamping of the power semiconductor component because here the current is determined essentially by the coupled load 4, as shown in FIG. 1A. The clamping of the logic component is relatively noncritical because it occurs in conjunction with a relatively high-impedance, current-limiting resistance RGND, as shown in FIG. 1A.
Therefore, it would be advantageous to specify a vehicle on-board electric power system with at least one power transistor which has more efficient clamping of the power transistors. Furthermore, it would be advantageous to provide a method for protecting a vehicle on-board electric power system by using an appropriately designed power transistor.